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Educational articles in General Biology

What are Stem Cells?

Stem cells are unique because they combine two key properties: Pluripotency
and self-renewal. Pluripotency means that the stem cells
can differentiate and give rise to multiple different cell
types. In addition, they have the ability to divide and
self-renew to maintain the original stem cell population.

There are two principally different types of stem cells:
Embryonic stem (ES) cells and adult stem cells (or
tissue-specific stem cells). In addition, induced pluripotent
stem (iPS) cells are very similar to ES cells, and are the focus
of intense ongoing research and development.

Embryonic stem (ES) cells arise from the fertilization of
an egg by a sperm. The first few rounds of cell division this
fertilized egg undergoes create both the extra-embryonic tissue
as well as a pool of identical ES cells that eventually will
give rise to the new individual. The unique ability of ES cells
to give rise to absolutely all cell types in the body has led to
increased interest in these cells for both basic and medical
research. Such research can improve our understanding of normal
development and genetic diseases, and also has a potential for
development of tissue regeneration therapy. However, there is
also controversy in regards to ethical issues when it comes to
the use of human ES cells.

Researchers have developed a method where mature differentiated
cells can be reprogrammed to become immature pluripotent cells,
named induced pluripotent stem (iPS) cells. These iPS
cells are found to be very similar to primary ES cells, but are
not identical. These cells provide an alternative source of
pluripotent cells, and iPS and ES cells therefore remain the
focus of intense research, both to understand the mechanisms of
pluripotency and to improve the method of reprogramming to
create iPS cells.

The 2012 Nobel Prize in Physiology or Medicine was
awarded to Sir John B. Gurdon and Dr. Shinya Yamanaka for "the
discovery that mature cells can be reprogrammed to become
pluripotent" (source: http://www.nobelprize.org/),
further emphasizing the importance of research on reprogramming
and induced pluripotency.

Adult stem cells are also referred to as tissue
specific stem cells and can give rise to all cell types
within the specific tissue. One example of an adult stem cell is
the hematopoietic stem cell (HSC). In adults, most HSC
are found in the bone marrow (BM), but can be immobilized into
the blood stream for instance for the purpose of hematopoietic
stem cell transplantation for treatment of leukemia. HSC are
pluripotent and give rise to red blood cells, platelets, and all
white blood cells required in the immune system. These stem
cells alternate between a quiescent (non-dividing, resting) and
a proliferative state (undergoing cell divisions), and provide a
life-long source of all blood and immune cells for the
individual.

The self-renewal property is crucial for life-long replenishment
of the downstream cell types. Exhaustion (or defective
self-renewal) of stem cells will lead to disease. For instance,
an exhaustion of the hematopoietic stem cells, which give rise
to all blood cells, will lead to anemia and immunodeficiency.
Anemia is the reduction of red blood cells that are crucial for
oxygen transport. Immunodeficiency is a defect in any immune
cell that is required to protect your body against infection,
damage and cancer.

However, the property of self-renewal is also potentially
dangerous, as uncontrolled self-renewal is a key feature of
cancer cells. Thus, in healthy stem cells, as well as other
diving cells, self-renewal is tightly regulated. A critical
combination of mutations can lead to loss of this regulation and
give rise to cancer. Control of self-renewal is also a major
concern and challenge that scientists face in regards to the
potential use of ES or iPS cells for tissue regeneration
therapy.

In summary, when talking about stem cells, it is
important to understand their unique properties and to
distinguish between three separate types of stem cells. 1: Embryonic
stem cells come from a fertilized egg and give rise to all
cell types of the body, but only exist in the early stages of
embryogenesis. 2: Induced pluripotent stem (iPS) cells
are created from mature cells by reprogramming. 3: Adult,
tissue-specific stem cells are maintained throughout our
lifetime and give rise to all cell types within a specific
tissue.

Note: For a schematic presentation of the
research that led to the 2012 Nobel Prize in
Physiology or Medicine, please see this
link.

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Latest Updates:

10/15-2012: Educational article: What are Stem Cells?

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